Migrate to GitHub Actions.

[jezdez]

Travis CI has a new pricing model which places limits on open source.

• https://blog.travis-ci.com/2020-11-02-travis-ci-new-billing
• https://www.jeffgeerling.com/blog/2020/travis-cis-new-pricing-plan-threw-wrench-my-open-source-works

Many projects are moving to GitHub Actions instead, including Jazzband projects:

• https://github.com/orgs/jazzband/projects/1
• https://github.com/jazzband-roadies/help/issues/206

This is based on https://github.com/jazzband/contextlib2/pull/26.

Python 3.9 support is also added.

TODO:

• [x] @jezdez to add JAZZBAND_RELEASE_KEY to the repo secrets.

[codecov[bot]]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@fa8d170). Click here to learn what that means. The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #226   +/-   ##
=========================================
  Coverage          ?   80.73%           
=========================================
  Files             ?       13           
  Lines             ?     1448           
  Branches          ?      233           
=========================================
  Hits              ?     1169           
  Misses            ?      214           
  Partials          ?       65           

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